import java.util.Scanner;

public class N_5 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
        int num = 0;
        int[] a = new int[26];
        for (char c : str.toCharArray())
            a[c-'a']++;
        for (int j : a)
            if (j % 2 == 1)
                num++;
        if(num == 0)
            System.out.println(1);
        else
            System.out.println(num);
    }
}
